This invention relates to acoustic well logging and, more particularly, to digitized full wave form acoustic well logging.
In recent years it has become desirable in the field of acoustic well logging, in order to provide more information about earth formations traversed by a well borehole, to digitize the full received wave form of an acoustic signal transmitted from an acoustic transmitter to an acoustic receiver over a relatively long spaced distance in the well borehole. For example, it is not uncommon in acoustic logging to have long spaced acoustic well logging sondes having the transmitter and receiver separated by distances of at least ten feet. This is in direct contrast to prior art systems which used relatively short spacings from exclusively acoustic transmitter and receiver pairs to perform acoustic velocity logging of earth formations in the vicinity of a well borehole.
In acoustic velocity logging it was not necessary to retain or to know the entire wave form of the acoustic signal received at each receiver. Rather it was desired only to detect the onset of the compressional wave or P wave which arrived at a receiver from a particular transmitter firing. The remainder of the acoustic wave form which contained shear waves and other acoustic propagation wave modes such as Raleigh waves or tube waves was considered relatively unimportant in the prior art.
It has been discovered, however, in relatively recent years that it is highly desirable to space an acoustic transmitter a relatively long distance from a plurality of acoustic receivers. Such a system can be used to record, in digital form, the acoustic wave forms arriving at each of a plurality of acoustic receivers. The long spacing between the transmitter and acoustic receivers allows time separation of the various modes of acoustic energy propagation to occur at the receiver due to the difference in transmission velocities of these different transmission modes in the earth formations and the borehole.
Previously, with short spaced acoustic transmitters and receivers, there was inadequate time separation of the various acoustic modes arriving at the receiver because the propagatin velocities were not sufficiently different to prevent interference at the receiver from one acoustic mode to another. However, using longer spaced acoustic transmitters and receivers has enabled the propagation of the waves over the longer distance to further separate in time the various acoustic modes of propagation due to the differences in propagation speed over the greater distance between the transmitter and receiver as these different modes of acoustic signals pass from the transmitter into the earth formations and along the boundaries of the well borehole and thence back into the distantly spaced acoustic receivers propagation speed difference effectively filter or separate the modes.
The more distant spacing of acoustic receivers from a transmitter, however, leads to further difficulties in obtaining a good quality acoustic well log. For example, acoustic transmitters must be made much more energetic than in the prior art in order to provide high signal to noise ratios of acoustic energy arriving at the more distantly spaced receivers. Moreover, so called road noise generated by the movement of the acoustic well logging sonde through the borehole and by electronic noise generated within the tool itself and other noises created by movement of the logging cable or vibrations being propagated down the logging cable or vibrations being propagated down the logging cable from the surface become more important in masking the relatively weaker arriving signals from the acoustic transmitter at each of the long spaced plurality of acoustic receiving transducers.
While it is true that information concerning the mechanical rock charcteristics of the earth formations can be derived from the full wave form acoustic logs of modern day acoustic long-spaced well logging instruments, it is also true that it is highly desirable to be able to still produce the so called Delta T log or log of travel time from the transmitter to an acoustic receiver of the compressional or P wave in the earth formation surrounding the well borehole. This is because a long known relationship has been developed for directly relating the acoustic travel time or velocity of propagation of P waves to the porosity of the earth formations in the vicinity of a well borehole. Particularly in very hard earth formations having fast propagation travel times, the acoustic P wave is carried or radiated away from the well borehole very efficiently when the transmitter is fired. This leads to relatively low amplitude arrivals of P waves at the distantly spaced acoustic receivers. P amplitudes are low only relative to later arrivals. If the entire waveform is to be recorded without clipping. The P waves appear small. P amplitudes in hard rock are still much larger than in soft or porous formations. Usually a technique involving the zero crossing detection of the oscillating acoustic wave form or the selection of a peak amplitude of first arrival is used for this purpose. However, the wide dynamic range requirement on the amplifiers and preamplifiers connected to the acoustic well logging receiving transducers become very stringent in these cases. Thus very low amplitude or relatively low amplitude road noise which occurs in the passage of the well logging instrument through the borehole can lead to severe difficulties in detecting the relatively low amplitude arrival of P waves in particularly in hard rock formations.
In the past it has been possible to make separate well logging runs using different relative amplification levels and different frequency filters tailored for the specific purpose of passing either relatively high frequency or relatively low frequency acoustic signals through the electronic systems to the digitizers. For example, in detecting the onset of the P wave arrivals, the high frequency components of the signal are the most desirable ones to detect. Relatively low frequency components can be shunted aside or filtered out for this purpose. That such filtering can be performed is due to the fact that most road noise caused by tool movement in the borehole or vibrational energy propagated down the well logging cable into the borehole occurs at much lower frequncies than the transmitted acoustic signal from the acoustic transmitter to the receivers. This is a much higher frequency than that of true naturally occurring noise due to movement of the well logging tool and cable in the borehole. The transmitter frequency is selected to put as much energy as possible into the borehole and to excite all modes of propagation. Lower frequency transmitters cause better excitation of Stoneley waves, etc. Lower frequency transmitters also mean road noise and signal frequencies can sometimes overlap, however, the bulk of road noise is below S signal frequency.